The long term objective is to determine the mechanism of transposition of the Escherichia coli transposon, Tn3. The two steps of transposition are cointegrate formation and resolution. Cointegrate formation involves duplication of the transposon and fusion of donor and recipient replicons via transposon bridges. It requires the transposase of Tn3 and host proteins. We hope to reproduce this process in vitro and have developed sensitive assays for the whole process and components of it. We have recently recloned transposase under transcriptional repressor control and will purify transposase and study its properties, particularly its binding to DNA. Cointegrate resolution requires only the resolvase of Tn3. We wish to determine the DNA winding changes that accompany resolution. We will try to determine the handedness of catenases produced by recombination and the untwisting that accompanies the process. We seek to trap recombinational intermediates and will test whether Holiday forms can be matured by resolvase. The path of DNA between recombinational sites is fixed during resolution and we will determine if this is accomplished by movement of resolvase between the sites. Resolvase is quickly becoming a paradigm not just for recombination but for binding of proteins to specific DNA sequences and we will study its physical properties and mode of binding to DNA. We have just begun to characterize inhibitors of cointegrate formation and resolution. These drugs should block the invasiveness of transposons that underlies their abilities to spread epidemically. We are interested in transposons because they have many important biological consequences including conferring antibiotic resistance and controlling activity of genes. RNA tumor viruses themselves seem to be a class of transposon. Also transposition presents a unique combination of specific replication and nonhomologous recombination and working out its mechanism promises insights into the processes.